1. Field of the Invention
The present invention relates to a frame rate conversion device using overtaking prediction, and an overtaking prediction method for use in the same. A video receiving display apparatus such as TV system is provided with the frame rate conversion device.
2. Description of the Related Art
In the broadcasting industry, the transition from analog terrestrial broadcasting to BS/CS/terrestrial digital broadcasting, so-called digitization of broadcasting, is currently proceeding. At the same time, the role of television is also greatly changing from “TV set placed in living room” toward “information window of home”. This trend of digitization increases the number of programs to be sent to each home (an increase in the number of channels), and the amount of information of video is also increasing from SD (Standard Definition) toward HD (High Definition).
The roles of display devices are also changing under this trend. Specifically, display devices having larger screen sizes and higher definitions are being developed so that video images with realistic presence can be reproduced in home living rooms. A typical example is a PDP (plasma display) of over 50 inches.
Under these trends toward digitization of broadcasting and larger screen sizes and higher definitions of display devices, as a method of using a television set as an “information window of home”, there is a form of display called multi-picture display, which simultaneously displays images from various media on one display. In general, a method of temporarily storing input sources having different frame rates in a frame memory, reading images from the memory in synchronism with the display rate of a display, and synthesizing and displaying the images on the basis of a picture layout is adopted to realize a multi-picture display function.
In this case, during the frame rate conversion of synchronizing different frame rates of the respective input sources having different formats with the display rate of the display, it is indispensable to prevent a disturbance of an image generally called “overtaking”.
The principle of occurrence of this overtaking and a general preventive method will be described below.
FIG. 11A is an explanatory view of overtaking in the case of input frame rate Fiv[Hz]>output frame rate Fov[Hz]. In the case where an input video image in which an automobile is running from the left to the right is updated in units of frame like (1)→(2)→(3)→(4)→(5) (represented by circled numbers in FIG. 11A, respectively), this video image is written to a frame memory in synchronism with the input frame rate Fiv[Hz] during periods of W0→W1→W2→W3→W4. The transition of a write address during this time can be represented as a repeated waveform such as the sawtooth waveform shown by solid lines in FIG. 11A.
In the meantime, reading from the frame memory is performed in synchronism with the output frame rate Fov[Hz] during periods of R0→R1→R2. The transition of a read address during this time can be represented as a repeated waveform such as the sawtooth waveform shown by dashed lines in FIG. 11A.
In this case, overtaking occurs at a point (marked with “OVERTAKING POINT” in FIG. 11A) where the write address and the read address cross each other. Accordingly, in an output video image, outputting is overtaken by the updating of input during the reading of one frame, and an image made of different upper and low frames (an old frame (2) on the upper side and a new frame (3) on the lower side) is outputted. It is possible to cope with this phenomenon by bringing the writing of the input video image to the frame memory to a stop during the period (W2) shown in FIG. 11B in which overtaking occurs, but the frame (3) is omitted from the output video image and a frame omission occurs.
FIG. 12A is an explanatory view of overtaking in the case of input frame rate Fiv[Hz]<output frame rate Fov[Hz]. In this case as well, overtaking occurs on the basis of a principle similar to that mentioned in connection with FIG. 11A. Accordingly, in an output video image, the updating of input is overtaken by outputting during the reading of one frame, and an image made of different upper and low frames (a new frame (3) on the upper side and an old frame (2) on the lower side) is outputted. It is possible to cope with this phenomenon by bringing the writing of the input video image to the frame memory to a stop during the period (W2) shown in FIG. 12B in which overtaking occurs, but the frame (3) is omitted from the output video image, and in this case, multiple frames are displayed in a superimposed manner (multiple display).
The frame omission and the multiple display which occur due to this overtaking preventing method cannot be avoided when frame rate conversion is performed. However, the case in which a video image contains images split within a frame due to overtaking causes an unnatural visual appearance, and in this case and in general, an overtaking measure can be taken to reduce the unnatural visual appearance.
In either case, by predicting overtaking in advance and bringing writing to the memory to a stop, it is possible to realize an overtaking measure in a single buffer (a memory capacity for one picture). In this overtaking measure using a single buffer, it is necessary to perform overtaking prediction, and a method for overtaking prediction is disclosed in JP-A-2001-13934.
In addition to the above-mentioned overtaking measure using a single buffer, a double-buffer overtaking preventing method is known. This method is a control method which prepares not a memory area for one picture but memory areas for two pictures and writes data to the memory areas while alternately switching the memory areas therebetween, and reads data from a frame different from a written frame so that overtaking does not occur.
If the frame rate conversion of the above-mentioned related art is to be applied to multi-picture display, the following two matters must be solved:
(1) reduction in frame memory capacity and simplification of overtaking measure control circuit; and
(2) handling of picture layout change during multi-picture display.
First, (1) will be described below.
Overtaking measures for multi-picture display must have a plurality of independent frame rate conversion processings because of the relationship between individual inputs and a display output.
Accordingly, if the related-art double-buffer or multi-buffer method is selected as a frame rate conversion method, a frame memory capacity of twice the number of multi-picture inputs becomes necessary. (For example, if a multi-picture display made of four pictures is to be handled, a memory capacity of eight frames becomes necessary.) Accordingly, the above-mentioned single-buffer method is more advantageous in that memory costs can be reduced. In this case, since overtaking measure control circuits are necessary by the number of pictures, it is required that the control method can be realized by a simple circuit.
Then, (2) will be described below.
In general, in the case of many single-picture displays, the timing formats of input and output are preset on a system basis, and is rarely dynamically changed. Accordingly, since frame rate conversion can be handled on predetermined fixed conditions, various related art methods including the constructions described in JP-A-2001-13934 and JP-A-2001-83928 are devised. However, when a picture from a certain image source is to be displayed in an offset state at an arbitrary position within the screen of a display device, like multi-picture display and reduced screen display, the picture layout is in many cases dynamically changed as a display mode for a user. Accordingly, since not only the timing formats of input and output differ from each other but also the frame rates of a plurality of inputs and a display output differ from each other, it is difficult to perform frame rate conversion under fixed conditions like single-picture display.
In other words, there is not a related art which takes into account the offset difference between an effective area and an input video effective area, so that accurate prediction of overtaking has heretofore been difficult.